Method and apparatus for protecting enclosed spaces



A. B. HUBE March 13, 1962 METHOD AND APPARATUS FOR PROTECTING ENCLOSED SPACES 2 Sheets-Sheet 11 Filed Nov. 17, 1959 A. B. HUBE March 13, 1962 METHOD AND APPARATUS FOR PROTECTING ENCLOSED SPACES 2 Sheets-Sheet 2 Filed NOV. 17, 1959 3:02. wmnmmwma 22x52.

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3,025,505 METHOD AND APPARATUS FGR PROTECTENG ENCLQSED SPACES Arthur B. Hubs, Huntington Station, N.Y., assignor to American District Telegraph Company, Jersey City,

N..l., a corporation of New Jersey Filed Nov. 17, 1959, Ser. No. 853,551 15 Claims. (Cl. 340-446) The present invention relates to electrical protection systems and more particularly to a method and apparatus for protecting enclosed spaces against intruders.

The invention is concerned with so-called space protection, as distinguished from perimeter protection in which the opening of a door or window is detected. Space protection systems generally are subject to interference from changes in ambient conditions which require maintenance of a reduce sensitivity in order to avoid spurious alarms. One of the principal objects of this invention has been to provide a space protection system which is relatively free from interference caused by changes in ambient conditions and hence can be operated so as to afford a high level of sensitivity.

The invention is particularly useful in the protection of pneumatic shelters, but, as will appear, the principles of the invention are also applicable to the protection of conventional structures.

Pneumatic shelters, often styled air houses are fabric enclosures completely supported by maintenance of an internal superatmospheric air pressure, usually of the order of 0.1 to 2 inches of water gauge. Such structures have been used for a wire variety of military and industrial purposes, such as covers for outdoor radar installations, portable warehouses, construction site covers and temporary shelters. Pneumatic shelters are supported by a continuously operating blower system which supplies air to the inside of the structure to maintain the necessary inflation pressure and to make up air lost through leaks and vents. Venting is necessary to prevent excess pressure buildup.

Protecting these pneumatic shelters against intruders, e.g., burglars, saboteurs, etc., by conventional means has not proven satisfactory. For example, ultrasonic systerns, which detect motion through Doppler effect on sound, depend upon air-borne sound energy for their operation. Consequently, when adjusted to overcome false-signalling conditions caused by air movements, as from the erecting blower in air houses, or by wind-caused movements of the structure, the ultrasonic system does not have adequate sensitivity to detect personnel movement. It then can be defeated by stealth.

Radio frequency or radar-type systems are usable only with an antenna layout which interferes with structure use, and even then it is possible to defeat such systems by slow motion attacks. Also, false alarm signals are initiated by movements external to the structure, and these systems do not define the limits of protection to the walls of the protected structure. Also, when missiles or other equipment are in place, peculiar shadow situations which affect the pattern of protection occur. Furthermore, the use of radio frequencies for any purpose around missile or radar installations should be avoided if possible. Another point is that jamming could occur. Lacing and foil, which constitute conventional burglar alarm methods for walls, windows, and similar structural surfaces, are entirely too difficult and expensive to apply to air-supported structures to be practical.

Vaults and safes and, to some extent, rooms have been protected by pressure-sensitive systems, as described, for example, in United States Patents 2,069,953 to Hopkins, 2,129,261 to .Chase et al., and 2,745,089 to 3,0255% Patented Mar. 13, 1962 Levy. While such systems may be used to advantage under the static conditions usually present in rigid structures, they are unsatisfactory for protection under the dynamic conditions encountered in pneumatic shelters. Thus, the static pressure in such shelters can be substantially affected by wind conditions and fluctuations in blower air flow. The inherent elasticity of such structures also makes monitoring of the static air pressure for protection purposes diflicult. Again, the internal volume of these shelters is usually so large that, having regard to normal blower characteristics, a small opening made by an intruder will have a relatively small and delayed effect on static air pressure which will be dithcult to detect with the sensitivity levels enforced by the need to avoid spurious alarms resulting from wind action.

The present invention affords a simple and eificient protection system eminently practical for protecting pneumatic shelters as well as other structures. Basically, the invention contemplates continuous monitoring of the make-up and venting air flows and the use of electrical signals proportional to these air flows to initiate alarm signals under appropriate conditions.

A principal object of the invention has been the provision of a novel and improved space protection system.

More particularly, it has been a principal object of the invention to provide such a system which is adapted for use in protecting pneumatic shelters as well as conventional rigid structures.

A further object of the invention has been the provision of such a system which is insensitive to external ambient conditions, either atmospheric, such as wind, snow, rain, lightning, temperature, etc., or man-made, such as passing aircraft or vehicles.

A further object of the invention has been the provision of a system of the above type which will respond rapidly to a penetration of the protected structure.

A feature of the invention has been the provision of a system of the above type which does not require a local electrical power supply for operation.

Another object of the invention has been the provision of a system of the above type which is simple, economical, easy to maintain and reliable.

Still another and important object of the invention has been the provision of a system of the above type which will exhibit extreme defeat resistance even in the face of skilled intruders such as highly trained espionage agents or saboteurs.

Yet another object of the invention has been the provision of a system of the above type which will fail safe, i.e., will register an alarm in the event of failure of a system component. Other and further objects, features and advantages of the invention will appear more fully from the following description.

Briefly stated, the method of the invention is concerned with the protection of an enclosed space which is normally maintained at a superatmospheric pressure by continuously supplying air to the interior of the space, the space being continuously vented to the external atmosphere through a vent opening of fixed size, the method comprising the steps of measuring the volume of air supplied to the space and the volume of air vented from the space through the vent opening, producing a first voltage proportional to the volume of air supplied per unit time, producing a second voltage proportional to the volume of air vented per unit time, these voltages normally having a fixed relationship relative to each other, comparing the first and second voltages, and using a departure from the fixed relationship between the voltages greater than a predetermined minimum to initiate an alarm signal.

The invention will now be described in greater detail with reference to the appended drawings, in which:

FIG. 1 is a schematic diagram illustrating one embodiment of the invention;

FIG. 2 is a schematic diagram illustrating another embodiment of the invention; and

FIG. 3 is a curve illustrating a typical blower characteristic for a pneumatic shelter.

Referring now to the drawings, and more particularly to FIG. 1, the reference numeral 10 refers to a pneumatic shelter, here shown in transverse cross section. The shelter it] will generally be made from a fabric, such as a high strength nylon coated on both sides with a synthetic elastomer compound, and the bottom peripheral edge will generally be anchored to the ground 11 or to a concrete base. An earth or other barrier may be provided to minimize leakage around the bottom edge, but this is not usually necessary. The shelter is completely supported by maintenance of a superatmospheric pressure in the space within the shelter. The required pressure is usually relatively small. For example, where only low velocity winds are encountered, a pressure as low as 0.1" H may be adequate. Where high wind velocities are likely to be encountered, e.g., in the range of 75 to 100 miles per hour or more, a higher internal air pressure will be needed, such as 1.0 to 2" H O, depending upon various constructional factors.

' Pneumatic shelters have been developed in a Wide variety of sizes and various shapes have been used, generally semi-spherical or semi-cylindrical. A typical shelter for warehousing purposes might be about 4-0 feet wide by about 80 feet long with a maximum height of 30 feet. Assuming a semi-cylindrical shape, such a structure would have an internal volume of about 113,400 cubic feet. Air losses will vary widely depending upon factors such as door sizes, average door open times, leaks, vent size, blower capacity and ratio of volume to surface area of openings. A typical structure of the type described having relatively small doors might require a blower capacity of the order of 1000 to 2000 cubic feet per minute. Such a blower, operated by a suitable electric motor, is shown at 12 and operates to force atmospheric air from the outside of the shelter It) to the inside space and to maintain the internal pressure at a suitable value. The blower 12 might be a centrifugal ventilating-type fan.

The output of blower 12 is discharged into the interior of the shelter through a cylindrical tube 13 which might be a part of the blower casing. Within the tube 13 is mounted a device for measuring the velocity of the air discharged by the blower into the space within the shelter i0, and hence also the volume of this air. This device might conveniently be a freely rotatable fan 14 mounted on a shaft 15 of a direct current generator 16. The shaft 15 will preferably be parallel to the axis of the tube 13. The generator 16 should be a small machine, for example, one of the type used as a tachometer generator. A small permanent magnet, ball-bearing, aviation-type motor is admirably suited for the purpose.

The output voltage of the generator 16 and the current flowing through the external circuit connected to its armature will be proportional to the speed of rotation of the fan 14 and hence will also be proportional to the velocity of the blower discharge air and to the volume of air discharged per unit time. In place of a fan, any convenient velocity-measuring device may be used, e.g., an 'annemometer mechanism, and in place of the generator 16 any suitable device for producing an electrical output proportional to the measured air flow may be used. However, a simple generator, as illustrated, is greatly preferred, especially from the point of view of reliability.

The output of generator 16 is supplied through a variale resistor 17 and a milliarnmeter 18 to one winding 19 of a difierential relay 20, which is preferably of the sensitive, 3-position, null type.

The shelter fit) is provided with a fixed area vent, here shown as a tube 21 passing through the ground 11 from the space within the shelter 10 to the outside atmosphere. A fan 22 mounted on the shaft 23 of a direct current generator 24 is mounted in the tube 21, the arrangement being similar to that provided in the blower output tube 13. The output of the generator 24 is supplied to the other winding 25 of differential relay 20 through a variable resistor 26 and a milliammeter 27. The relay 20, resistors 17 and 26, and the milliammeters 18 and 27 may conveniently be installed in a housing indicated by the dashed line 23. The housing 28' will preferably be protected with tamper contacts, as is conventional in protection systems.

With the shelter it) inflated and with all movable openings closed, the resistors 17 and 26 should be adjusted so that the current flow through milliammeter 18 resulting from blower make-up air flow acting on fan 14 and the current flow through milliammeter 27 resulting from air flow through vent 21 are equal. In this way current flow through each of coils 19 and 25 will be equal and differential relay 20 will be balanced. Under this condition, relay armature 28 will be in a mid-position between contacts 29 and 39. Under normal protection-on conditions, the blower output air flow and the vent air flow will each be substantially steady. The blower air flow will normally not be large because the blower will be operating on the upper end of its characteristic curve against a relatively high back pressure. The vent air flow will be relatively large (comparing the small vent size to the large blower output size). By protection-0n conditions is meant the condition in which all movable openings are closed and detection of any peneration of the shelter by way of doors, walls or otherwise is desired. Should it be desired to maintain unequal current flows in the two relay coil circuits, the coils of the relay 20 may be selected to provide a null condition with some current ratio other than 1:1. In this case, the resistors 17 and 26 will be adjusted accordingly to provide a corresponding current ratio under normal protection-on conditions.

Armature 28 and contacts 29 and 30 are connected to signalling equipment at some remote point through conductors 31, 32 and 33, respectively. Such remote point will preferably be the central station of a central station electrical protection system or a guard post or station of a proprietary electrical protection system. A local alarm signal, e.g. a hell or light, may be provided in addition to or in lieu of such a remote signalling connection.

If the current output of either of the generators 16 and 24 exceeds the current output of the other by a predetermined value (depending upon the selected sensitivity of differential relay 20), armature 28 will make with one or the other of contacts 29 and 30, completing corresponding respective electrical circuits. This occurs since armature 28 will remain in its mid-position only when the current flow through coils 19 and 25 is substantially equal. By substantially equal current flows is meant that the ratio between these currents lies in a predeter mined narrow range.

Any opening of appreciable size which is made in shelter 16 will result in a decrease of the air flow through the vent '21. By opening of appreciable size is meant a passageway affording means of entry for an intruder from the outside of the shelter 10 to the interior thereof. Such an opening might be made by opening a door, cutting a slit in the wall, digging a tunnel under the periphery of the wall bottom edge or lifting the bottom edge of the wall. In any event, however the opening may be made, if it be sufiiciently large to afford admittance to an intruder, the volume of air carried by the vent 21 will immediately decrease, resulting in a corresponding decrease in the output of vent generator 24 and a corresponding decrease in the current flow through coil 25 of relay 20. The result will be to unbalance the relay in a sense to cause the armature 28 to make with the contact 30. Whether or not such making is effected solely as a result of the decrease in vent air flow will depend, of course, on the size of the opening relative to the size of the vent and other leakage paths which may be present.

By reference to FIG. 3, which is a typical curve of internal shelter pressure versus blower air flow, it will be seen that a decrease in internal air pressure will result in an increase in blower air flow. Thus, assuming normal blower operation at point A, when the intruder effects an opening in the shelter structure, the internal pressure will decrease. The increase in shelter leakage and consequent decrease in air flow through the vent occur substantially instantaneously, but because of the large air volume involved, the decrease in air pressure will be manifested relatively slowly. However, as the pressure drops the blower will tend to operate at a lower point on its characteristic curve, e.g., at the point B. But the blower air output at this point is greater, so that the output of generator 16 will be greater, in turn increasing the current flow through winding 19 of differential relay 20. Since the blower is operating near the high end of its characteristic curve, even a very small change in pressure will produce a fairly substantial increase in blower air flow, so that, for practical purposes, a substantial change in blower air flow will occur in a time interval sufficiently small as to be considered simultaneous with the decrease in vent air flow.

An increase in current flow through winding 19 will affect the armature 28 in the same way as a decrease in current flow in coil 25. Hence, when an intruder makes an opening, the resulting decrease in vent air flow and increase in blower air flow will have an additive effect in unbalancing the relay 2t) and causing armature 28 to make with contact 30. Because of this additive effect, a very small opening made in a large pneumatic shelter can easily be detected, and an opening large enough to admit a person can be detected even though it remains open only for a very brief time, such as the time required to open a door, enter and close the door.

Armature 28 and contact 30 are included in an external electrical circuit which registers alarms, and the registration of the alarm can be effected in any way desired and at any place desired. Through the use of an additional contact, a break as Well as a make alarm circuit can be controlled by the armature 28, or a break only circuit may be used.

Should the blower 12 fail, as by an interruption in its power supply, the output of generator 16 will drop to zero almost immediately, unbalancing the relay 20 in the opposite direction so that armature 28 makes with contact 29. Armature 28 and contact 29 are included in another external electrical circuit. But closing of this other circuit by making of armature 28 and contact 29 may be taken as a trouble signal rather than an alarm, should it be desired-as it usually will be--to differentiate between blower trouble and intruder alarm conditions. It should be observed that, while blower failure produces an almost immediate and very large effect on generator 16, the effect on generator 24 will be small and delayed. Of course, a slow decrease in the output of generator 24 will be observed as the volume of air flowing through vent 21 decreases owing to reduced internal air pressure in shelter 10, but this decrease in generator 24 output will be negligible from the standpoint of retaining relay 20 balanced. Hence a trouble signal will certainly be sent if the blower fails.

Should any failure occur in the circuit of coil 19, as by failure of generator 16 or breaking of fan 14, relay 20 will be unbalanced and a trouble signal will be transmitted by the making of armature 28 with contact 29. Should any failure occur in the circuit of coil 25, relay 6 20 will be unbalanced, and an alarm signal will be transmitted by the making of armature 28 with contact 30. Thus the protection system will fail safe.

Should normally open alarm and trouble circuits be used, as shown in FIG. 1, supervision of circuit integrity may conveniently be afforded through maintaining a small supervisory current fiow through these circuits. Resistors 34 and 35 connected between conductors 31 and 32 and between conductors 31 and 33, respectively, will permit such small supervisory current flows, but will not materially affect the alarm or trouble current flows which take place upon unbalance of relay 20.

The effect of wind on a pneumatic shelter is a complex phenomenon. When gusts of wind strike the shelter, there is a tendency to increase the static pressure, and, in the case of sharp gusts, this is a significant factor which renders the use of pressure sensitive protection devices very difficult unless the sensitivity be maintained at a very low level, which, of course, is undesirable. The static pressure increase resulting from wind is promptly reduced by a number of factors. First, the whole structure tends to roll on its base so as to avoid marked distortion of its flexible sides. Second, as the wind indents one side the opposite side will subsequently bulge in self compensation, and so the net volume change will not be as large as. might be anticipated from the indentation caused by wind force. Third, the Wave-like nature of the wind causes the wind effect to occur in the form of traveling ripples rather than a general flattening of the entire windward side.

In so far as the detection system is concerned, the effect of wind is to produce a momentary increase in pressure which is quickly compensated by an adjustment in shape of the flexible sides to maintain a constant volume. But, since the detection system of the invention operates on air flow measurement rather than pressure measurement, the inertia afforded by the large volume of air involved prevents substantial changes in air flow before the pressure change is compensated. Thus a high degree of system stability is provided which reduces the spurious alarm problem, in turn permitting maintenance of a high sensitivity.

Referring now to FIG. 2, there is shown a modification of the construction of FIG. 1. However, in FIG. 2 most of the elements correspond to elements in FIG. 1 and are designated by like reference numerals. In FIG. 2 the generators 16 and 24 are of the alternating current type, and full wave rectifier circuits 36 and 37 are interposed between generator 16 and coil 19 and between generator 24 and coil 25, respectively. In FIG. 1, a balanced current flow through coils 19 and 25 results in armature 28 being in a mid-position out of contact with both contacts 29 and 36. In FIG. 2, the relay contacts are arranged as transfer contacts. Thus, in FIG. 2 a balanced current flow through coils 19 and 25 causes armature 23 to make with back contact 30. Un'balance of the relay coil currents greater than a predetermined minimum but in either direction will cause armature 28 to make with front contact 29, opening the normally closed circuit of conductors 31 and 33 and closing the normally open circuit of conductors 31 and 32. With this arrangemerit, a blower failure will result in the same signal indication as an alarm, but the opening of one circuit followed by the closing of the other affords the double drop signal frequently used in central station electrical protection service. Since one circuit is normally closed, constant supervision of the integrity of the conductors is afforded.

In FIG. 2, the vent 21 is located adjacent the top of the pneumatic shelter rather than at the base, as in FIG. 1, but the operation is the same except for the different relay contact operation, as described.

The arrangements illustrated in FIGS. 1 and 2 are simple, reliable, eificient and easy to maintain, and thus are admirably suited to the provision of electrical protection. Hence, these system represent the preferred embodiments of the invention. Other equipment could be used in practicing the method of the invention if for any reason it were desired to use other types of equipment. For example, the output frequencies of the generators l6 and 24 of FIG. 2 will be proportional to the speed of armature rotation. Accordingly, these frequencies could be compared, as by mixing, and the resultant beat frequency, either sum or difference, could be used to initiate an alarm if it fell outside a given range, as by the use of a band pass filter having as a pass band the given permissible range of deviation of the beat frequency.

A flow-sensitive vane in the blower output tube and a similar vane in the vent tube could be used to position potentiometer sliders to in turn produce voltages which can be compared in any desired way to initiate an alarm whenthe comparison shows greater than a predetermined difference.

In one practical embodiment of the invention, using the construction and circuit illustrated in FIG. 2, the pneumatic shelter protected was generally semi-cylindrical in shape, about 60 feet long, 18 feet wide, and 14 feet high. This shelter was supported entirely by air at a pressure of A to /2 ounce per square inch as provided by a one-horse power blower. The vent was a 9 diameter pipe 2 feet long. The generators provided equal output currents (as adjusted by variable resistors in the load circuits) lying in the range o-f'l to 10 milliamperes. The normal generator output currents, as measured by milliammeters 18 and 27, were each found to vary approximately :0.2 milliampere under strong gusty wind conditions. Opening of a 25 square inch aperture in the wall of the pneumatic structure resulted in a combined current change of approximately 1.0 milliampere. Thus, the current excursion resulting from opening a 25 square inch area-which is far too small to permit entry into the protected areawas more than twice the maximum combined excursion resulting from gusty wind action. For the particular example set forth, the differential relay could have been set to operate the armature 28 on a total current change of about milliamperes for both coils. However, even a higher permissible current change would have been usable since it would not normally be necessary to detect an opening as small as 25 square inches. It will be evident that the invention is capable of affording great stability combined with sensitive. detection.

The invention has been described principally in connection with pneumatic shelters since it affords a means for providing reliable, efiicient and simple electrical protection for such shelters which, so far as is known, cannot be duplicated by other electrical protection systems. However, the principles of the invention are alsoapplicable to advantage to the protection of rigid structures, e.g., conventional rooms, vaults, etc. While such conventional structures can be protected efiiciently by many of the well-known protection systems, including space protection systems, the principles of the invention afford a space protection system having the great advantage of being inherently insensitive to changes in ambient conditions. For this reason, the invention will, under appropriate circumstances, be usable in preference to other forms of space protection.

In using the invention for protecting a rigid structure, it will be necessary to employ a continuously operating blower which builds up a superatmospheric pressure within the structure and a vent which allows a continuous fiow of air from within the structure to the outside atmosphere. The blower and vent will preferably be arranged so that-having regard to air leakage paths other than the ventthe blower will operate near the high end of its characteristic curve.

While the invention has been described in connection with specific steps and specific embodiments thereof, various modifications of the method and apparatus described will occur to those skilled in the art without departing from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

l. The method of detecting intrusion into an enclosed space which is maintained at a superatmospheric pressure through the provision of a continuous supply of air under pressure, air being continuously vented from said enclosed space through a vent opening of fixed size, comprising the steps of measuring the volume of air being supplied to said space and producing a first electrical quantity Whose magnitude is proportional to the flow rate of said supplied air, measuring the volume of air being vented from said space through said vent opening and producing a second electrical quantity whose magnitude is proportional to the flow rate of said vented air, said first and second electrical quantities normally having a fixed relationship lying Within a given range, comparing the magnitudes of said first and second electrical quantities, and producing an alarm signal indication when the relationship between the magnitude of said first and second voltages lies outside said given range.

2. The method of detecting intrusion into an enclosed space which is maintained at a superatmospheric pres sure through the provision of a continuous supply of air under pressure, air being continuously vented from said enclosed space through a vent openingof fixed size, comprising the steps of measuring the volume of air being supplied to said space and producing a first voltage, whose magnitude is proportional to the flow rate of said supplied air, measuring the volume of air being vented from said space through said vent opening and producing a second voltage whose magnitude is proportional to the flow rate of said vented air, said first and second voltages normally having a fixed relationship lying Within a given range, comparing the magnitudes of said first and second voltages, and producing an alarm signal indication when the relationship between the magnitudesof said first and second voltages lies outside said given range.

3. The method of detecting instrusion into an enclosed space comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings, converting said velocity measurements into electrical quantities each proportional to the rate of flow of air through a respective one of said openings, comparing said electrical quantities, said electrical quantities normally having a predetermined relationship lying within a given range, and producing an alarm signal indication when the relationship between said electrical quantities lies outside said given range.

4. The method of detecting intrusion into an enclosed space, comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings, converting said velocity measurements into electrical voltages each proportional to the rate of flow of air through a respective one of said openings, comparing said voltages, said voltages normally having a predetermined relationship lying within a given range, and producing an alarm signal indication when the relationship between said voltages lies outside said given range.

5. The method of detecting intrusion into an enlosed space, comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings,

' converting said Velocity measurements into electrical voltages each proportional to the rate of flow of air through a respective one of said openings, impressing each of said voltages on a respective circuit, adjusting the impedances of said circuits so that the ratio of the currents flowing therethrough as a result of said impressed voltages will lie within a given range under normal conditions, comparing said currents, and producing an alarm signal indication when said ratio lies outside of said given range.

6. The method of detecting intrusion into an enclosed space, comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings, converting said velocity measurements into electrical voltages each having a magnitude proportional to the rate of flow of air through a respective oneof said openings, impressing each of said voltages on a respective circuit, adjusting the impedances of said circuits so that the ratio of currents flowing therethrough as a result of said impressed voltages will lie within a given range under normal conditions, comparing the magnitudes of said currents, producing a first distinctive alarm signal indication when said ratio lies outside of said given range in one direction, and producing a second distinctive alarm signal indication when said ratio lies outside of said given range in the other direction.

7. The method of detecting intrusion into an enclosed space, comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings, converting said velocity measurements into electrical voltages each having a magnitude proportional to the rate of flow of air through a respective one of said openings, impressing each of said voltages on a respective circuit, adjusting the impedances of said circuits so that the ratio of currents flowing therethrough as a result of said impressed voltages will be substantially unity, causing said currents to flow through respective windings of a differential relay, and using an unbalanced condition of said relay to produce an alarm signal indication.

8. The method of detecting intrusion into an enclosed space, comprising the steps of continuously supplying air to said enclosed space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said enclosed space through a second opening of fixed size, measuring the velocity of the air flowing through said first and second openings, converting said velocity measurements into electrical voltages each having a magnitude proportional to the rate of flow of air through a respective one of said openings, impressing each of said voltages on a respective circuit, adjusting the impedances of said circuits so that the ratio of currents flowing therethrough as a result of said impressed voltages will be substantially unity, causing said currents to flow through respective windings of a differential relay and using the unbalanced condition of said relay resulting from preponderance of one current over the other to produce a first distinctive alarm signal indication, and using an unbalanced condition of said relay resutling from preponderance of the other current over said one current to produce a second distinctive alarm signal indication.

9. The method of supervising a pneumatic shelter Which is supported substantially entirely by maintenance of a superatmospheric pressure in the space within the shelter, comprising the steps of supplying air to said space through a first opening of fixed size to maintain the air in said space at a superatmospheric pressure, continuously venting said space to atmospheric pressure through a second opening of fixed size, continuously measuring the rate of flow of air through at least one of said openings, converting said measured quantity into an electrical quantity proportional to said rate of flow of air, and using a predetermined change in said electrical quantity to produce an alarm signal indication.

10. The method of detecting intrusion into an enclosed space which is maintained at a superatmospheric pressure through the provision of a continuous supply of air under pressure, air being continuously vented from said enclosed space through a vent opening of fixed size, comprising the steps of measuring the volume of air being supplied to said space and producing a first voltage whose magnitude is proportional to the flow rate of said supplied air, measuring the volume of air being vented from said space through said vent opening and producing a second voltage whose magnitude is proportional to the flow rate of said vented air, supplying said first and second voltages to respective electrical circuits adjusted so that the resulting currents flowing therethrough under normal conditions will have a ratio lying within a given range, comparing the magnitudes of said currents, and producing an alarm signal indication when the ratio between the magnitudes of said currents lies outside said given range.

11. The method of detecting intrusion into an enclosed space which is maintained at a superatmospheric pressure through the provision of a continuous supply of air under pressure, air being continuously vented from said enclosed space through a vent opening of fixed size, comprising the steps of measuring the volume of air being supplied to said space and producing a first voltage whose magnitude is proportional to the flow rate of said supplied air, measuring the volume of air being vented from said space through said vent opening and producing a second voltage Whose magnitude is proportional to the flow rate of said vented air, supplying said first and second voltages to respective electrical circuits adjusted so that the resulting currents flowing therethrough under normal conditions will be substantially equal, comparing the magnitudes of said currents, and producing an alarm signal indication when the ratio between the magnitudes of said currents departs from unity by more than a predetermined amount.

12. Electrical protection apparatus for supervising an enclosed space having a first opening of fixed size through which air is supplied continuously under pressure to maintain said space at a superatmospheric pressure and a second opening of fixed area through which air is continuously vented, comprising first and second velocity measuring devices each arranged in a respective one of said openings so as to be responsive to the velocity of the air flowing therethrough, individual means associated with said devices for producing first and second electrical quantities each proportional to the rate of flow of air through a respective one of said openings, comparing means coupled to said individual means for comparing said electrical quantities, and alarm signalling means operatively associated with said comparing means and arranged to produce an alarm signal indication when the compared relationship between said electrical quantities lies outside a given range.

13. Electrical protection apparatus for supervising an enclosed space having a first opening of fixed size through which air is supplied continuously under pressure to maintain said space at a superatmospheric pressure and a second opening of fixed area through which air is continuously vented, comprising first and second rotatable devices each disposed in a respective one of said openings and arranged to be rotated at a rate proportional to the velocity of the air flowing through said respective openings, first and second electrical generators operatively connected to said first and second devices, respectively, and each arranged to produce an electrical quantity proportional to the rate of rotation of the corresponding rotatable device,

a comparison circuit coupled to said generators for comparing said electrical quantities, and alarm signalling means operatively associated with said comparison circuit and arranged to produce an alarm signal indication when the ratio of said electrical quantities lies outside of a given range.

14. Apparatus as set forth in claim 13 in which said comparison circuit comprises a differential relay having two coils each of which is coupled to a. respective one of said generators.

15. Electrical protection apparatus for supervising an enclosed space having a first opening of fixed size through which air is supplied continuously under pressure to maintain said space at a superatmospheric pressure and a second opening of fixed area through which air is continuously vented, comprising first and second rotatable device each disposed in a respective one of said openings and arranged to be rotated at a rate proportional to the velocity of the air flowing through said respective openings, first and second electrical generators operatively connected to said first and second devices, respectively, and each arranged to produce a voltage proportional to the rate of rotation of the corresponding rotatable device, a differential relay having a pair of operating coils, circuit means intercoupling each of said coils with a respective one of said generators, said circuit means including an adjustable element for adjusting the ratio of the currents flowing through said coils whereby said ratio may be set within a predetermined range for normal conditions of said enclosed space, and alarm signalling means operatively associated with said relay and arranged to produce an alarm signal indication when the ratio of the currents flowing through said coils falls outside said predetermined range.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,261 Chase Sept. 6, 1938 

